Field
The disclosed technology relates to a device and a method of acquiring an image. In particular, the image may be acquired using time delay and integration.
Description of the Related Technology
Time delay and integration (TDI) image sensing is a popular technique used for inspection of scenes with inherent linear motion such as in machine vision or earth observation. TDI image sensing may be particularly useful in light starved applications with low irradiance on the focal plane, typically a consequence of imaging faint objects and/or high speed imaging requirements, since integration of the low irradiance levels is allowed. In such circumstances, a good signal to noise ratio (SNR) in acquiring the image is important in order for the detected light not to be drowned by noise. A TDI sensor is therefore often implemented in charge-coupled device (CCD) technology, as this offers essentially noiseless signal integration in the charge domain.
In TDI image sensing it may often be of interest to record spectral content of the detected light for each imaged point of an object. A detector system may thus be provided with a dispersive element for splitting different wavelengths of the incoming light. Alternatively, different pixels on the image sensor may be associated with different filters in order to separate detection of different wavelengths. However, acquiring images with a high spectral and spatial resolution implies that a large amount of data needs to be gathered and a sensor system needs to be adapted to allowing gathering of data in three dimensions (two spatial dimensions and one spectral dimension).
This implies that substantial data rates are required and causes a high computational load in order to allow extraction of the information of interest from the gathered data.
Alternatively, a filter profile may be tuned to the needs of a particular application. However, this may be achieved by fixing the filter profile during a camera production process, making such types of cameras very inflexible and prohibitively expensive. Alternatively, a filter may be changed in front of an image sensor, but this would typically result in costly and/or bulky devices.
International Application WO 2003/106917 discloses a TDI technique. A two-dimensional focal plane array is divided into sub-arrays of rows and columns of pixels, each sub-array being responsive to light energy from a target object which has been separated by a spectral filter into a predetermined number of spectral bands. Each sub-array has its own read out channel to allow parallel and simultaneous readout of all sub-arrays of the array. However, this device is quite bulky and it takes a long time to acquire spectral information from a portion on the target object, since the portion needs to be moved over each sub-array of the array to acquire the spectral information.
U.S. Pat. No. 6,678,048 discloses a programmable optical filter for use in hyperspectral imaging using TDI is disclosed. The filter is used together with a complementary metal-oxide-semiconductor (CMOS) image sensor. A variable gain feature allows individual rows of pixels to be attenuated individually, such that a focal plane spectral processing of an image is provided. Thus, computational requirements in post-processing of the acquired image are greatly reduced. However, the system uses a CMOS image sensor, which implies that noise is added in each transfer of detected light onto the next row in the TDI array.